In agricultural machines, such as telescopic loaders, wheel loaders or front loaders on tractors, it is known practice to use a hydraulic spring support system that provides spring support for the boom or the rocker in order to achieve overall improved spring suspension comfort on the vehicle, particularly during operation. To this end, the lifting side of the hydraulic cylinder is connected to a hydraulic accumulator by means of an appropriate hydraulic arrangement of valves in order to provide spring support through the hydraulic accumulator. Furthermore the lowering side of the hydraulic cylinder is connected to a hydraulic reservoir in order to prevent cavitation on the lowering side on one hand and to allow free movement of the piston rod during the spring support process on the other hand. To improve safety against a sudden lowering of the boom or the rocker, these spring support systems can be equipped with load holding valves or automatic shut-off valves to secure the hydraulic cylinder against ruptured hoses. However it is then necessary, in order to permit a lowering of the hydraulic cylinder, to close the reservoir connection on the lowering side of the cylinder so that sufficient pressure can be built up in order to open the load holding valve. Only after opening the load holding valve can oil flow from the lifting side of the hydraulic cylinder
In EP 1157963 A2 a spring support system is proposed for the boom of a telescopic loader, which is provided with a load holding valve or an automatic shut-off valve in order to secure the boom against lowering. In order to affect a pressurized lowering of the boom on one hand, which requires the load holding valve to be opened, and provide a spring support function also in the neutral position of the hydraulic cylinder on the other hand, a separate control valve is provided. This control valve has to be closed in order to close a connection to the reservoir established for the spring support and to be able to build up the pressure in the supply line required to open the load holding valve. In the case of the spring support system disclosed in EP 1157963 A2, however, sudden lowering of the boom may occur as a result of different pressure loads in the hydraulic cylinder and the accumulator when a spring support position is engaged. Varying loads of this type can occur when the boom is loaded for example in the raised position and then the spring support position is engaged. This establishes a sudden connection, while different loads are present in the accumulator and the hydraulic cylinder. For safety reasons it is therefore necessary to lower the boom completely before activating the spring support function.